The present invention relates to transflectance-type fetal pulse oximetry probes. "Transflectance" probes attach to a single physiological surface. Their light source and detector need not lie on opposite sides of the pulsatile tissue. "Transmission" probes, by contrast, monitor signals across pulsatile tissue, for example signals passing from the dorsal to the volar surface of a finger. The invention relates to improvements for transflectance probes, especially to enhance the light signal in the presence of fetal hair or other light-attenuating materials when the probe is attached to the head.
Application Ser. No. 264,196, here incorporated by reference, describes various embodiments of an improved perinatal pulse oximetry probe. The function of the probe and the nature of pulse oximetry are discussed in that application and will not be repeated here. The application particularly refers to "curved surface portions" through which the optical signals pass. When brought into contact with the fetal tissue, these curved portions create a dimple in the tissue surface.
The dimple helps prevent light from shunting between the source and the detector without passing through blood-perfused tissue. It also improves the contact between the probe and the fetal tissue. Nonetheless the oximetry readings, if made on the head, may be inaccurate or inconsistent due to low light levels, i.e., poor signal-to-noise ratio. The optical signal levels may be attenuated by intra-uterine materials like blood, dead cells, mucous, vernix caseosa, or fetal hair. These materials and fetal hair collectively will be referred to as "substances."